Elevated pressure product, such as oxygen and nitrogen, produced by the cryogenic rectification of feed air is increasing in demand due to such applications as coal gasification combined-cycle power plants.
One way of producing elevated pressure product from a cryogenic rectification plant is to compress the products produced by the plant to the requisite pressure. However, this approach is costly both because of the initial capital costs and because of the high operating and maintenance costs for the compressors.
Another way of producing elevated pressure product from a cryogenic rectification plant is to operate the plant columns at a higher pressure. However, this puts a separation burden and thus a recovery burden on the system because cryogenic rectification depends on the relative volatilities of the components and these relative volatilities are reduced with increasing pressure.
One way for sustaining the separation of feed air at elevated rectification pressures is feeding the largest possible portion of the feed air into the higher pressure column of a double column air separation plant. This achieves the maximum amount of high purity nitrogen reflux that the conventional double column arrangement can attain. However, at sufficient pressure levels this method will not be sufficient to avert significant reductions in oxygen recovery.
Another way for sustaining the separation of feed air at elevated rectification pressures is the utilization of heat pump compression loops. In such methods one or more low pressure streams are recycled through additional compression equipment and the compressed flow is returned to the column system to further drive the separation. Such systems are complicated to operate efficiently and are also costly depending upon the specific compression equipment employed.
Accordingly, it is an object of this invention to provide a cryogenic rectification system which can operate at elevated pressure with improved recovery over that attainable with conventional high pressure systems.